The present invention relates to spool valve devices, such as employed in pneumatic control valves for railway freight cars and particularly to the high pressure spool valve whose main function is to connect emergency reservoir air to the car brake cylinders during an emergency brake application and thereby provide a higher emergency brake pressure than the maximum attainable service brake pressure.
While it is generally accepted that spool type valves are desirable from the standpoint that a substantially leak free operation can be economically achieved over a long service life, it is also recognized that the pressure seals that make the foregoing possible may also introduce a friction component that causes resistance to spool valve actuation. In the industry standard, ABD/W type freight brake control valves, and in newer ABDX valves, the high pressure spool valve device is designed to provide, in addition to the main function of providing high pressure during emergency, the function of trapping service brake pressure on the inshot valve piston during emergency to assure the same inshot brake pressure when making an emergency on top of a service application as when making a straightaway emergency; and the quick action chamber blowdown function to delay recharging the train brake pipe pressure following an emergency brake application for a period of time sufficient to assure a complete halt of the train and thereby prevent premature release of the emergency brake application.
In combining these several functions in a single spool valve, numerous O-ring seals are provided on the spool to prevent cross leakage between the several spool valve grooves via which the appropriate pressure connections are made. Consequently, the high pressure spool valve device may be subject to somewhat high breakout resistance to valve actuation, from both its release and emergency application positions, due to the relatively high breakaway friction between these O-ring seals and the spool bushing with which the O-rings are sealingly engaged.
For example, the high pressure spool valve is pressure actuated from its normal release position to an application position in response to an emergency brake application to provide the above-mentioned functions; and in its application position is actuated by a return spring to reset the valve to its normal release position. It will be appreciated that prior to such actuation to either application or release position, the spool valve sits in a static condition in which O-ring friction and thus frictional resistance to valve actuation is maximum. Accordingly, the return spring must be relatively strong to overcome this static friction and assure reset of the spool valve from application to release position. The force exerted by this relatively strong return spring in release position, however, counteracts the pressure actuating force during an emergency to such extent that emergency actuation of the high pressure spool valve under very low pressure conditions can be impeded.